Rheology is branch of physics dealing with the deformation and flow of matter. It is particularly concerned with the properties of matter that determine its behavior when a force is exerted on it. Thus, it is concerned with the study of the change in form and flow of matter, embracing viscosity, elasticity and plasticity. The present application is directed to the subset of fluid dynamics concerned with the flow of fluids, primarily liquids in Newtonian and non-Newtonian regimes. Rheological relationships can provide a direct assessment of processability, are useful for monitoring and controlling a process, are a sensitive method for material characterization (such as changes to the molecular weight), and useful for following the course of a chemical reaction or changes to a fluid in simulated conditions. Rheological measurements allow the study of chemical, mechanical, thermal effects, effects of additives, or the course of reaction byproducts. All measurements of viscosity involve imparting motion to a fluid and observing the resulting deformation of that fluid.
Viscosity is a physical property that characterizes the flow resistance of a fluid. It has been defined as a measure of the internal friction of a fluid where the friction becomes apparent when a layer of fluid is made to move in relation to another layer. It is the resistance experienced by one portion of a material moving over another portion of the material. Viscosity is commonly used to characterize petroleum fluids, such as fuels and lubricants, and often they are specified in the trading and classification of petroleum products. Kinematic viscosity for petroleum products is commonly measured in a capillary viscometer by a standard method such as the ASTM D445 standard. The ASTM D445 standard involves measuring the time for a fixed amount of liquid to flow under gravity through a calibrated glass capillary under a reproducible driving head and a closely controlled temperature. In practice, this method has some challenges due to size limits of the apparatus, due to geometry, the relative sample size, and difficulty in changing shear rates. In addition, the calibrated glass tubes are fragile, difficult to clean, and relatively expensive. Therefore, it is undesirable to use a capillary viscometer for samples which would tend to diminish the repeatability of the capillary tube for example by coating the tubes, since these tubes would need to be removed and cleaned or disposed of prior to reuse. Changing glass capillary tubes in the ASTM D445 standard is a cumbersome and delicate procedure with a process delay since the replacement glass capillary and temperature bath must come to equilibrium.
Viscometers commonly are separated into three main types: Capillary, rotational and moving body. Most of these viscometers can produce viscosity measurements at a specified constant shear rate. Therefore, in order to measure the viscosity over a range of shear rates, one needs to repeat the measurement by changing the parameters (such as height, capillary tube dimensions) for capillary tube viscometers, by changing the rotating speed of the cone or cup in rotating viscometers, or changing the density of the falling object in the moving body viscometer.
The capillary tube viscometer has been principally defined by the Hagen-Poiseuille Equation especially for Newtonain fluids. In a Newtonian fluid the shear stress is proportional to the shear rate, and the proportionality constant is called the viscosity. To measure viscosity with a capillary tube viscometer, the pressure drop and flow rate are independently measured and correlated to some standard fluid of known viscosity. The three general types of glass capillary viscometers most frequently used include the modified Ostwald types for transparent liquids (Cannon-Fenske routine), the suspended level type for transparent liquids (Cannon-Ubbelohde types) and the transverse-flow for transparent and opaque liquids (British Standard BS U-tube reverse flow). While there are precise instructions for operating each of the above capillary viscometers, generally all follow the same set of basic steps. The test sample is inserted into the viscometer and temperature controlled. After reaching test temperature the test sample is allowed to flow under gravity past two timing marks with time recorded on the calibrated capillary tube. Thus, the driving force is the hydrostatic head of the test liquid. The viscosity is calculated as a product of the flow time and the calibration constant. External pressure can be applied to many of the capillary viscometers to increase the range of measurement to enable the study of non-Newtonian behavior.